This is from gov.uk.

The Criminal Justice Act 2003 defines serious harm as: ‘death or serious personal injury, whether physical or psychological’.

OASys defines serious harm as ‘an event which is life threatening and/or traumatic and from which recovery, whether physical or psychological, can be expected to be difficult or impossible’.

Most people's understanding of what is a serious offence is determined by the harm caused, but it can also include designated offences, like those under the Serious Crimes Act. For example, possession of a firearm causes no harm to anyone, but is still included in the list of offences designated by UK law as serious crimes.

It's one of those words which is bandied about by politicians, without any clear understanding of what it actually means.

But one of the reasons why the govt chose to set the limit for a two thirds early release at 7 years was the impact on prison numbers. It was originally going to be 4 years, but they consulted and were told that putting it at 4 years would mean thousands more prisoners locked up for longer. They changed it to 7 years so it would affect fewer prisoners. So their understanding of what serious means, in that context, is serious enough to justify keeping those people in prison.